1. Field of the Invention
This invention relates to a light modulating device suitable for an optical recording apparatus, an optical display apparatus or the like.
2. Description of the Prior Art
Using a light flux to effect recording or display has heretofore been widely practiced. Various techniques of imparting modulation to a light flux for this purpose are known, and U.S. Pat. No. 4,281,904 discloses varying the electric field distribution in crystal having an electro-optic effect and causing a light flux entering a portion of the crystal in which the refractive index is varied as a result of the electric field distribution to be diffracted, thereby accomplishing modulation. However, electro-optic crystal is expensive and in use, a predetermined polarizing characteristic must be given to the light flux entering the crystal. Also, where the above-mentioned modulation is to be effected, in order that the light flux may be totally reflected in the interior portion of the optical crystal material in which an electric field has been produced and that the diffracting efficiency may be enhanced, a limitation that the light flux must be caused to enter as nearly parallel to the electrode as possible is imposed.
U.S. Pat. No. 3,612,653 may be mentioned as another example of the prior art. In the embodiment shown in this patent, an electrically conductive thin film is formed on a support member and this thin film is disposed so as to be in contact with liquid. A ray light entering the support member obliquely from the support member side is transmitted to the liquid side when the thin film is not supplied with electric power while, on the other hand, the liquid near the thin film changes into vapor when the thin film is supplied with electric power and as a result, the incident light ray is totally reflected in the interface between the electrically conductive thin film and the vapor. Therefore, the above-described embodiment suffers from numerous disadvantages, in that it is necessary to apply a high voltage to the electrically conductive thin film in order to change the liquid into vapor, the degree of freedom of the structure and arrangement of the device is lost to keep the vapor bubbles in contact with the thin film and achieve the stailization of the function of the device because the vapor bubbles are affected by buoyancy, and the electrically conductive thin film is liable to be destroyed by the localized pressure when the vapor bubbles disappear and this leads to a problem in respect of the durability of the device.
In contrast, in recent years, attention has been paid to the utilization of the distribution of refractive index by the thermal effect to effect modulation of light. The modulation of light utilizing the distribution of refractive index by the thermal effect is introduced in "The Deflection of Light Caused by the Variation in Refractive Index by Heat" (Nikkei Electronics, Aug. 16, 1982) or "Thermo-optic Deflection and Switching in Glass" (Applied Optics, Vol. 21, No. 19, pp. 3461-3465).
In these examples, TiO.sub.2 crystal or glass made by the use of the ion-exchange process is used as the thermal effect medium. Generally, the degree of temperature dependence of the refractive index of a solid is small and to obtain a desired deflection characteristic, a high voltage is required as the voltage applied between electrodes or the voltage applied to a heater resistance. Also, to obtain an efficient deflection characteristic in each of the above-described examples, it is necessary to limit the propagation position of the incident light flux relative to the position of the electrode or the heater. That is, since the degree of temperature dependence of the refractive index is small as previously mentioned, it is necessary to cause the light flux to propagate through a portion as near to the position of the electrode or the heater as possible and as parallel to the surface of the electrode or the heater as possible in order to provide a suitable phase variation to the light flux.
As a further example of the prior art, an example in which use is made of an optical element comprising a dielectric metal oxide film on a light-transmitting type substrate and a pair of electrodes on the metal oxide film and the variation in refractive index by a variation in temperature of the dielectric metal oxide film is utilized to deflect transmitted light is described in Japanese Laid-open Pat. Application No. 23019/1983. However, again in this example, use is made of a solid material whose degree of temperature dependence of the refractive index is small and therefore, to obtain a desired deflection characteristic, a high voltage is required as the voltage applied between electrodes.
Japanese Laid-open Patent Application No. 97026/1983 may be mentioned as still a further example of the prior art. In the embodiment shown in this patent application, the interface between a non-heated area and a heated area is inclined relative to the optical path and a light ray is refracted and deflected in said interface by a variation in refractive index in the heated area. However, again in this embodiment, dielectric crystal is used as the medium of the heated area and, therefore, the amount of variation in refractive index resulting from the temperature change of the heated area is small, and this leads to the disadvantage that the angle of deflection is limited. Further, in the embodiment shown in said patent application, due to its construction in which a heat-generating resistance member is formed on the surface of the dielectric substrate which is parallel to the incident light, the area of the dielectric substrate in which a variation in refractive index occurs is limited to the vicinity of the heat-generating resistance member and as a result, the incidence position of the incident light ray is limited.
Also, as a prior art example of the total reflection element using liquid crystal, there is a report entitled "Integrated Hologram Memory Device Using Liquid-Crystal Optical Switch Matrix", M. Sawada et al., a paper presented at the Technical Group on Optics and Quantum Electronics, IECE Japan, Jan. 1982, available as OQE81-116. In this report, with attention being paid to the extraordinary refracting property of liquid crystal, an electric field is applied to liquid crystal to thereby vary the orientation of the liquid crystal and vary the refractive index thereof. However, this prior art example suffers from the disadvantages that the driving voltage is limited to an AC voltage, a very high voltage is required where a planar-type electrode is used, and where sandwich type electrodes are employed, two electrodes are opposed to each other on the opposite sides of the liquid crystal, which leads to a limitation in construction of the device.